From West German patent application No. 26 07 721 (which corresponds to U.S. Pat. Nos. 4,110,525, 4,112,210, 4,148,985, 4,148,986 and 4,152,505) it is known to prepare a homopolymer of a conjugated diene or a copolymer of a conjugated diene with another conjugated diene or with a vinyl aromatic compound having extremely different microstructures and a monomodal distribution of the molecular weights by means of a catalyst system formed of the reaction product of:
(a) an organic compound of a metal of group 3A of the periodic classification of elements of the Mendeleev Table having one of the following formulas: EQU M.sup.1 M.sup.3 R.sup.1 R.sup.2 R.sup.3 R.sup.4 EQU M.sup.2 (M.sup.3 R.sup.1 R.sup.2 R.sup.3 R.sup.4).sub.2 EQU M.sup.3 R.sup.1 R.sup.2 R.sup.3 EQU M.sup.1 OM.sup.3 R.sup.1 R.sup.2
in which M.sup.1 represents an alkali metal, M.sup.2 represents an alkaline earth metal, M.sup.3 represents a metal of group 3A, R.sup.1, R.sup.2, R.sup.3 represent an alkyl or aralkyl radical and R.sup.4 represents either an alkyl or aralkyl radical or a radical XB in which X represents an oxygen, sulfur or nitrogen atom and B represents an alkyl or aralkyl radical or a radical M.sup.3 (R.sup.5 R.sup.6) in which R.sup.5, R.sup.6 represent an alkyl or aralkyl radical,
with (b) at least one electron-donor compound containing at least one heteroatom selected from the group consisting of aprotic polar compounds, protic polar compounds and compounds formed of the reaction products of protic polar compounds with an alkali metal or with an alkaline earth metal.
It is desirable to have means which make it possible to modify and regulate the distribution of the molecular weights of the homopolymer of a conjugated diene or a copolymer of a conjugated diene with another conjugated diene or with a vinyl aromatic compound for a number of industrial uses of these products, since the modification of the molecular weight distribution makes it possible to improve greatly certain properties such as, for instance, the machineability, the cold flow, the raw coherence, the raw tackiness, etc., without penalizing the other properties.
It is known to the man skilled in the art that it is possible to broaden the molecular weight distribution and obtain bimodal or multimoldal polymers (e.g., EPT polymers) by mixing together several polymers of different viscosity.
However, such a process has the drawback of requiring the separate synthesis of several polymers of different viscosities, which results in problems of reproducibility of the process, requires very large quantities of catalyst, results in long periods of time and finally makes this process uninteresting both from a technical standpoint and from an economic standpoint.
It is also known to modify the molecular weight distribution of homopolymers and copolymers in processes carried out either batchwise or continuously by breaking up the amount of catalyst necessary and adding it at different times during the course of the homopolymerization or copolymerization. However, such a manner of operation, which also requires very large amounts of catalyst, which are larger the greater the desired broadening of the molecular weight distribution is, is therefore also very expensive. Furthermore, it would be extremely difficult to carry out industrially.